Tower Grounding

[MICHAEL.ZEUG at x400gw.ameritech.com]

    While it is correct to say that the concrete base of a tower does 
    contribute to the overall ground for the tower, it is important to 
    remember that the real purpose of the concrete base is to support 
    the tower!  Anything that compromises the ability of the base to do 
    its primary function represents the potential for the entire tower 
    to fail.
    
    The average lightning stroke has a peak current on the order of 
    40,000 amps.  A direct hit on a tower that is only grounded through 
    the concrete base results all of the current flowing through the 
    base.  Concrete, even after it has cured, contains some water.  The 
    ligntning strike passing through the concrete can cause this water 
    to turn to steam.  As the water expands it can cause cracks in the 
    concrete.  Worst case the concrete will "explode" much like a tree 
    will during a direct hit.  However, even if the concrete doesn't 
    explode, the damage done by the lightning strike could significantly 
    weaken the base and in turn its ability to support the tower.
    
    For this reason separate ground rods (or other grounding techniques) 
    should be used to shunt the lightning energy around the concrete 
    base.
    
    Mike Zeug K9EC
    Michael.Zeug at ameritech.com